Fiber ceramic composites and method of producing same

ABSTRACT

A HIGH TEMPERATURE RESISTANT STRUCTURAL MATERIAL COMPRISING FIRST AND SECOND RIBBONS OF SOLID VITREOUS MATERIAL HAVING A THICKNESS BETWEEN APPROXIMATELY 0.001 AND 0.005 INCH, AND A LAYER OF FIBERS INTERMEDIATE SAID RIBBONS, THE RIBBONS HAVING PORTIONS WHICH SURROUND AND WET THE FIBERS AND WHICH ARE FUSED TOGETHER TO UNITE THE RIBBONS.

April 2%, W71 A. c. SIEFERT ETAL 3,575,78Q

FIBER CERAMIC COMPOSITES AND METHOD OF PRODUCING SAME Filed Dec. 27. 1966 BILAYER ELEMENT,

ELEVATED TEMPERATURE 41/6057 6 J/mmr & 6950 Z JET/.5

INVENTORfl' ATTORNEYS United States 3,575,789 FIBER CERAMIC COMPOSITES AND METHOD OF PRODUCING SAME August C. Siefert, Granville, and Fred T. Sens, Newark,

Ohio, assignors to Owens-Corning Fiberglas Corporation Filed Dec. 27, 1966, Ser. No. 604,974 Int. Cl. B32b 3/26, 5/18 U.S. Cl. 161-193 19 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to new and improved fiber-ceramic composites, and a new and improved method of producing the same; and more particularly to such materials having superior strength at elevated temperatures where most metals deteriorate.

The aviation and aerospace industries have long sought lightweight structural materials which would have better strength than do the metals and metal alloys that have been commercially available. These industries have need for such a material for use in parts in the turbine sections of jet engines, as well as for airframe components of high speed aircraft and re-entry vehicles.

Much research has been done in an attempt to develop materials having the requirements above described. Industrial research facilities as well as the governments of the United States and foreign countries have investigated the strength of a great number of alloys and inorganic materials, and much has been written on the properties of the materials investigated. A number of inorganic fibers have been found to have high tensile strength in the region where metals melt or deteriorate, including the region where the high melting metals, such as tungsten, lose strength. Some of these fibers having these amazing properties are silicon carbide, boron carbide, silicon nitride, beryllium oxide, graphite, and alumnum oxide whiskers. In addition, certain metals and metalloids which oxidize or sublime in air have been found to have superior strength, if protected from the air, in an intermediate range of temperature between the range where currently available metals can be used and the range where only in organic materials of the type above described still have strength. An example of a metalloid having a desirable strength to weight ratio, in this intermediate range of temperature, is boron. Boron, however, oxidizes readily in air at these elevated temperatures and, therefore, can only be used in applications where it is protected from the air.

The high strength inorganic fibers mentioned above, and which have been investigated so far, have not been produced in monolithic structures haping appreciable strength. The high strength fibers or crystals, are sometimes called whiskers because of the manner in which they are grown. There has been heretofore no way in which the superior properties of short fibers could be harnessed or effectively used to produce structural members and to our knowledge short whiskers have never successfully been used as a reinforcement. A great amount of research has been done in trying to reinforce other materials with fibers of intermediate lengths, but none heretofore has been particularly effective. By way of example, fibers of these materials have been mixed with inorganic cements, but these materials crack easily when subjected to mechanical or thermal shock. The result has been, that the fibers do nothing more than hold together a material that otherwise 3,575,789 Patented Apr. 20, 1971 cracks or deteriorates at the high temperature service conditions.

According to the present invention, structural materials are produced wherein fibers of metals and/or inorganic materials are coated with a highly vitreous ceramic, such as glass, a glass ceramic, or porcelain like ceramic, and the coated fibers are fused and pressed into a structural member. The combination of ceramic and fibers which are used are preferably chosen so that the ceramic has a smaller coefficient of expansion than the fibers, so that the ceramic will be placed under compression when the fused structural member cools to room temperature. Glasses for example, and the other ceramics which are useful are usually subject to breakage by shock at room temperature. When placed under compression by the fibers, however, it has been found that they can be subjected to quenching and mechanical shock without breaking. It has been found that the composites of the invention do not shatter when subject to thermal or mechanical shock. At low temperatures, the fibers hold the ceramic in compression, and it appears that this prevents shattering. At elevated temperatures, the ceramic becomes less brittle, and even though the fibers do not hold the ceramic under compression to as great a degree as at low temperature, no shattering occurs. Since the method of manufacture, taught by the present invention involves a cooling from the temperature of fusion of the ceramic, there will still be a small amount of compression of the ceramic at temperatures below its solidification point, and at which the composites will be normally used.

An object of the present invention is the provision of a new and improved structural material wherein high strength metallic and/or inorganic fibers are bonded together by a ceramic that is preferably a vitreous ceramic which may or may not have some devitrification.

Another object of the invention is the provision of a new and improved material of the above described type in which the fibers hold the ceramic in compression.

Another object of the invention is the provision of a new and improved method of producing such composites on a continuous large scale basis.

Another object of the invention is the provision of a new and improved bi-layered material having an inorganic matrix which is non-electrically conductive and which will flex as a result of temperature change.

Another object is the provision of new and improved electrical windings which will withstand high temperatures without deterioration of the insulation on the windings.

Further objects and advantages of the invention will become apparent to those skilled in the art to which the invention relates from the following description of several preferred embodiments described with reference to the accompanying drawings forming part of this specification, and in which:

FIG. 1 is a somewhat diagramatic three dimensional view showing one embodiment of apparatus for producing a composite of the present invention;

FIG. 2 is a partially diagramatic three dimensional view similar to FIG. 1 showing another embodiment of apparatus;

FIG. 3 is a fragmentary isometric view of one embodiment of the composite of the present invention;

FIG. 4 is a diagramatic three dimensional view of apparatus for producing contoured bodies of the present invention;

FIG. 5 is an isometric view of a bilayered composite of the invention which deflects upon change in temperature; and

FIG. 6 is an isometric view similar to :FIG. 5, but showing the bilayered composite in a deflected form.

The apparatus shown in FIG. 1 generally comprises at least two, or preferably three or more, glass melt tanks 10, having bushings (not shown) in the bottom of each tank for extruding thin ribbons or sheets 12 of glass. Each sheet 12 of glass is pulled downwardly by suitable means not shown to attenuate the thin molten films of glass into ribbons having a thickness of approximately 0.002 inch. This thickness may vary, but is preferably between 0.001 and 0.005 inch. The ribbons 12 of glass are exposed to ambient conditions and quickly solidify to form a vitreous, or solid solution phase and each ribbon 12 passes around a roller 14 and then proceeds in a horizontal direction. A pressure roll 16 is positioned beneath each of the rollers 14 so that each ribbon is confined between a pair of rolls 14 and 16. At least one roll of each pair is preferably driven to advance the ribbon passing therethrough, and to in some instances, provide the necessary pull for attenuation of the ribbon. In the embodiments shown in FIG. 1, the ribbons are advanced to the right as seen in FIG. 1, and the ribbon 12 of the left tank passes through the bite of the pair of rolls 14, 16 for the ribbon 12 issuing from the second melt tank. The juxtaposed ribbons 12 from the left and middle melt tanks passes to the third pair of rolls 14, 16 where the ribbon from the right hand melt tank is brought down upon the top surface of the pair of ribbons to form a stack of three juxtaposed ribbons. The left, middle, and right glass melt tanks 10, the ribbons 12, and pairs of roller 14 and 16, are identical; and will be distinguished from each other where necessary by the sufiix L, M, and R, respectively.

The apparatus of FIG. 1 is adapted to form composites of ceramic and aligned short fibers, or whiskers. The composite is produced by providing a layer of aligned short fibers sandwiched between a pair or ribbons. This is done by the apparatus generally designated 18. The fiber aligning apparatus 18 comprises a hopper 20 in which randomly oriented fibers are dumped. A slotted opening. not shown, in the bottom of the hopper 20 allows the fibers to fall unto the upper end of a slightly inclined vibrating table 22 having a serrated upper face. The upper end 20 is wider than the ribbons 12 by a considerable amount, and the serrations 24 converge to a total width at the lower end of the table which is slightly less than the width of the ribbons 12. The depth of the serrations 24 adjacent the upper end of the table is greater than at the lower end of the table, and the vibratory movement of the table 22 causes the randomly oriented fibers 26 which issue from the hopper 20 to become aligned lengthwise of the serrations 24. The construction of the table 22 is such as to make the fibers proceed towards the lower end of the table 22 at an increasing rate. The fibers at the upper end of the table 22 will lay several fibers deep in each serration. As these fibers are accelerated, the fibers are spread apart lengthwise of the serrations. As previously indicated. the serrations decrease in height and converge at the bottom end of the table, so that the fiber issue from the table in a generally uniform layer not more than approximately 2 fibers deep. As previously indicated. the apparatus is depicted generally schematically in FIG. 1, and although FIG. 1 for clarity shows an appreciable thickness to the bottom edge of the table 22, it is in reality very thin, so that there is substantially no free fall of the fibers onto the ribbons 12. There is, therefore, a uniform layer of fibers laid down upon each ribbon which thereafter passes beneath another ribbon 12 in a sandwich-like arrangement. Thereafter another layer of fibers is laid down upon the second ribbon, and a third ribbon is brought down upon the second layer of fibers. The process can be repeated to provide a stack of any desired number of ribbons and layers of fibers, so long as the stack consists of at least two ribbons and one layer of fibers positioned therebetween. The tables 22, shown in FIG. 1 are identical and are supported by a vibratory structure 28 that is electrically driven by a motor, not shown.

After the sandwich of ribbons and fibers is produced, it passes through a furnace in which there is a heater which initially fuses the side edges of the ribbons together to prevent lateral movement of air between the ribbons.

Thereafter, heat from a main heating element is applied across the remaining width of the stack or ribbons to uniformly raise the ribbons to a temperature at or above their softening point. In the embodiments shown in the drawing, the heat necessary for fusing the side edges and the main body of the ribbons is provided by a natural gas burner 30 having upstream portions 32 for sealing the side edges, and a downstream main portion 34 for fusing the remainder of the ribbons. Immediately, thereafter, and while the glass is still in a softened state, it passes beneath heated compression rolls 36 whose surface temperature is at or near the softening point of the glass. The surface of the rollers is purged with a stream of a reducing gas to prevent adhesion of the glass to the heated rolls. It is believed that the surface tension of glass in a reducing atmosphere is greater than when in an oxidizing atmosphere, and that this helps prevent the Wetting of the rolls by the glass. The compression provided by the pair of rolls 36 squeezes the softened vitreous material between and around the individual fibers, to wet the fibers and cause the glass from one ribbon to fuse with the glass of another ribbon. Thereafter, the composite moves out of the heated area of the furnace and cools to room temperature. The heat supplied by the burner 34 and rolls 36 bring the fibers to approximately the same temperature as the glass. The fibers preferably have a coefiicient of expansion greater than that of the ceramic material of which the ribbons are composed, so that upon cooling from the temperature of fusion of the ceramic, the fibers place the ceramic material under compression. The composite for-med is shown generally in FIG. 3.

In most instances, it will be desired to obtain as high a loading of the fibers in the composite as is possible, and there will usually be enough glass in the ribbons 12 themselves to flow around and between the fibers and still leave a covering of an appreciable thickness on all sides of the composite. In other instances, however, it may be desired to add additional ceramic material between the ribbons and along with the fibers. This can be done by spreading a thin layer of the ceramic material either on top of the fibers, or on top of the plate 22 adjacent the lower edge thereof. This ceramic material may be a powder of the same composition as the ribbon, or can be of a different composition which will modify the composition of the ribbons after being heated and compressed therewith. One good material is a dried and powdered silica sol such as powdered Ludox. A vibratory hopper is provided at 38 for sprinkling a suitable powdered material onto the fibers for this purpose. In some instances it will be desirable to add a hard particulate material, such as particles of boron, and the vibratory hopper can be used for this purpose. In other instances, it may also be desirable to lay a thin layer of an inert gas purge on top of the fibers before a ribbon is brought down upon the fibers. A gas purge apparatus is shown generally at 40. In still other instances, it may be desirable to add a small amount of a carbonate, as for example sodium carbonate, potassium carbonate, lithium carbonate, etc. with a silica sol to liberate carbon dioxide and also provide a cation for altering the composition of the glass. The carbon dioxide liberated helps to purge air from between the fibers.

The apparatus shown in FIG. 2 corresponds to that generally shown in FIG. 1 and differs principally therefrom, in that the fibers are continuous, and are supplied from beams 42 similar to that used in the textile art. The beams 42 are made by simultaneously winding a large number of fibers onto a roll. These fibers are in generally touching parallel alignment, and are brought down in position between the ribbons so oriented. Those portions of the embodiment shown in FIG. 2 which are similar to corresponding portions of the embodiment shown in FIG. 1 are designated by a like reference numeral characterized further in that a sulfix a is a affixed thereto.

The continuous fibers may be of any suitable type as for example glass, steel, stainless steel, or boron, etc.

and may be coated or uncoated. In some instances, it may be desirable to coat the fibers with a liquid silica sol to bind the fibers together so that they remain in an aligned sheet or layer. A silica sol, of course, can be dried prior to being wound into a roll to hold the fibers together. Other types of liquid silicates can be used, as for example water glass or modifications thereof that include other cations, or a silica sol that has been modified with other cations. These materials will modify the composition of the ribbons when fused therewith to form the composite.

In some instances, it will be desirable to delay fusing the ribbons together until the stacks are pressed or otherwise deformed so that the fibers will remain in proper orientation. In other instances, it will be desirable to provide a small amount of a liquid binder to hold the fibers in position between the ribbons. A silica so], such as Ludox, can be applied to hold the fibers in position between the ribbon. The application of the silica sol can be accomplished in any suitable manner as for example by means of a spray header 43 shown in FIG. 2. In many instances, it will be desired to provide a finished composite that is thicker than the stack produced from the apparatus shown in FIGS. 1 and 2. Where this is desired, the stack can be sheared into a plurality of identical lengths and these lengths stacked and fused into a thick monolithic structure.

In the apparatus shown in FIG. 4, the stack produced on the apparatus shown in either FIGS. 1 or 2 is sheared into lengths that are superimposed on each other to form a pile. A desired thickness of sheets are slid off of the pile onto a press having upper and lower heated dies 44 and 46, respectively. After the sheets are in position over the lower die 46, a heating element 48 is brought into position and the ceramic material heated to a working temperature above the softening point of the ceramic. Thereafter, the heating element is removed and the upper die is brought down upon the lower die to provide a stamped article having the configuration shown at 50.

The composites of the present invention can be made into the usual standard structural shapes by hot bending and/ or welding. As for example, T sections can be formed by fusing the side edge of one sheet to the major face of another sheet, utilizing the glassy phase of the sheets, or an additional solder glass to fuse the sheets together. If the glass has a fusion point below that of the composites, the vitreous material of the composites will be held in compression during the soldering or welding operation, with the result that the operation can be carried out without substantial cracking or shattering of the composite. In most instances, however, it will be desirable to heat the finished weldment in an annealing furnace to equalize stresses. After annealing the weldment it can be cooled in a uniform manner using a cooling rate which either prevents devitrification or provides controlled devitrification. It will be understood that a controlled devitrification as occurs in Pyroceram (a commercially available glass ceramic) will be beneficial in many instances, in that it provides a slight amount of crystallization throughout the vitreous phase to in some instances enhance strength and other properties.

In most instances, it will be desirable to make the weldments and/or structural sheet material from a ceramic of uniform composition in order that distortion will be held to a minimum during heating and cooling periods of the composite. In still other instances, however, it may be desired to provide a structure which will undergo controlled fiexing. This can be accomplished as shown in FIGS. 5 and 6 by producing a sheet material using two layers of different ceramic composition, each of which is preferably reinforced by fibers and each of which can be produced by either of the methods shown in FIGS. 1 or 2. Two stacks of dissimilar ceramic materials can be brought together and fused by the same general process indicated in FIGS. 1 and 2, and can be formed into a curved configuration as shown generally in FIG. 6. Upon cooling from the fusion temperature, the composite will assume an essentially flat configuration as shown in FIG. 5. Thereafter, the material will flex to positions intermediate the conditions shown in FIGS. 5 and 6 as the temperature of the composite is increased. Such a material will have use as a thermostatic element for use at high temperatures, and will be electrically nonconductive, particularly when the composite is made from short noncontinuous fibers. Applications for such a material will readily occur to those skilled in the art.

Now that physical arrangements of the composites of the present invention have been described, many uses for this new type of material will occur to those skilled in the art. It will be understood that the ceramic and fibers of the composite can be selected on the basis of the intended use of the composite. As for example, where the use is such that very little expansion should take place during a change in temperature, the ceramic used can be a low expansion borosilicate or Pyrex glass. Where higher temperature resistance is desired, aluminum silicate glasses or S Glass can be used as the vitreous ceramic. Where very high temperature resistance is required, essen tially pure silica or quartz can be used. The fibers which are used will also depend on the intended use. Since Pyrex has a softening point between the low melting glasses and the high melting quartz, steel fibers, or stainless steel fibers can be used with Pyrex. Aluminum silicate and borosilicate glasses can be reinforced with boron fibers to form composites which have light weight, high impact strength, high fiexural strength, and high modulus of elasticity to density ratios, even at temperatures up to 1600 F. Because the glass totally surrounds each of the fibers, oxidation of the fibers does not occur. Aluminum silicate glass, boro-silicate glass, S Glass and quartz glass have very good resistance to chemical attack from most materials, and have exceedingly good durability, with respect to water. Borosilicate glass is extremely corrosion resistant particularly in acid or neutral conditions, and can be reinforced with steel or stainless steel fibers to produce composites having corrosion resistance, high impact strength, and high fiexural strength up to temperatures of C.

It will be desirable, and in some instances necessary, to provide a controlled oxidation of the metal fibers to increase the weta-bility and adhesion of the fibers by the matrix material. Controlled oxidation can be accomplished, by avoiding the purging of all air from the fibers prior to fusing the ribbons around the fibers. In still other instances, it may be desirable to provide a controlled oxidation by heating the fibers to a predetermined preheat temperature in air prior to being covered by a ribbon. This can be accomplished by a preheat burner 52 that is preferably located beneath the ribbon on which the fibers are oriented. By so doing, the lay of the fibers is not disturbed, and if the preheater is a gas flame, its products of combustion will not dilute the oxidizing atmosphere adjacent the fibers.

The present invention makes possible such a large number of composites of differing materials that it is not feasible or possible to describe the precise conditions which should be used in forming the various possible combinations of fiber and ceramic. It is believed that the types of control described above will allow those skilled in the art to readily proceed to the optimum conditions for producing a composite of any desired combination of materials. It will further be understood that the various pieces of equipment utilized in the processes shown and described, are patentable in and of themselves, and for this reason are treated diagramatically herein.

A table of some of the properties of some of the composites which have been produced thus far will now be given. These composites are of the general nature shown in FIG. 3. The same properties given will exist in struc tures fabricated by various processes and techniques. Complicated shapes, if properly made from these composites, will have substantially the same properties.

Flexural modulus, P.S.I. Izod impact strength it.-lb./in. diameter Room Pure Pure ceramic, ceramic, "J Not 20% not 20% tcmp.At 1,000F. at 1,000 F. notched notch notched 23 10 17. 5X10 23 10 5. 7 3. 9 0. l9 0. l3 18X10 16X10 1.37 0.19 14X10 12X10 14Xl0 0.19 0.13

1,000 Room F.

TABLE I Composite ficxurnl strength, p.s.l.

Room

temp.

after Size, Density, Room At in. gms./cc. temp. 1,000 F. 2.35 160, 000 140, 000

Percent Fiber type Orientation 30 Monofilament. UnidirectionaL 0. 0015 i3 1:333:13::::::::::: i8::::::::3:: 3%? weight: 28.7 SiO 11.7 N2 0, 9.1 GaO, 17.2 B30, 26.3 B 0 5.3 Z110, 3.1 F

Percent Fiber type LT-38 (Pyrex) 70 Boron as formed, oxide surface. Do. 70 Boron etched and nitrided X-243 glass L 70 Type-304 stainless steel- I X-243 glass has the folio ing composition by Ceramic While the invention has been described in considerable detail, we do not wish to be limited to the particular embodiments shown and described, and it is our intention to cover hereby, all novel adaptations, modifications, and arrangements thereof which come within the practice of those skilled in the art to which the invention relates.

We claim: 1. The method of forming structural materials that are resistant to high temperature, said method comprising: continuously advancing a first ribbon of a ceramic material in a given direction, continuously laying a layer of fibers on said ribbon of ceramic material, continuously advancing a second ribbon of ceramic material in said given direction over the top of said layer of fibers with 1 the edges of said ribbons generally superimposed, passing the superimposed edges of said first and second ribbons of ceramic material past localized heating areas which fuse at least said edges together, passing said ribbons past localized heating areas which fuse the area of said ribbons between said edges, and compressing said ribbons around said fibers While said ribbons are in a state of fusion.

2. The method of claim 1 wherein said superimposed edges are sealed a sufficient distance ahead of the remainder to provide a U-shaped enclosure which restricts gas flow over the fibers and provides controlled oxidation of the surface of the fibers.

3. The method of claim 1 wherein the atmosphere between said ribbons is controlled so that it is essentially neutral at the point where the major portion of the ribbons are fused together.

4. The method of claim 1 followed by superimposing a multiplicity of ribbons, heating the ribbons to a temperature causing fusion of the ceramic materials without producing fusion of said fibers, and compressing said ribbons together while said ceramic is in the state of fusion.

5. The method of forming structural materials that are resistant to high temperature, said method comprising:

continuously advancing a first ribbon of a ceramic material in a given direction, continuously laying a first layer of aligned fibers on said ribbon of ceramic material, continuously advancing a second ribbon of ceramic material in said given direction over the top of said layer of aligned fibers, continuously laying a second layer of aligned fibers on said second ribbon, continuously advancing a third ribbon of ceramic material in said given direction over the top of said second layer of aligned fibers, passing the superimposed layers and ribbons past a localized heating area which produces fusion of said ceramic ribbons, and compressing said ribbons around said fibers while said ribbons are in a state of fusion.

6. The product produced by the method of claim 1.

7. The structure of claim 6 wherein said ribbons are 5 aluminosilicate glass and said fibers are boron fibers.

8. The material of claim 6 wherein said ribbons are of materials having different coefficients of expansion.

9. The structure of claim 6 wherein said ribbons are a devitrified glass.

10. The structure of claim 6 wherein said ribbons are a devitrified glass and said fibers are carbon.

11. The structure of claim 6 wherein said ribbons are a borosilicate glass and said fibers are boron.

12. The structure of claim 6 wherein said ribbons are 5 essentially pure silica and said fibers are silicon carbide.

13. The structure of claim 6 wherein said ribbons are essentially pure silica and said fibers are aluminum oxide.

14. The method of claim 1 including the step of applying a water solution of a silicate to said layer of fibers before moving the ribbons and layer of fibers through the heating area which fuses the ribbons together.

15. The method of claim 1 wherein the fibers are aligned short Whiskers fed to the ribbon by a vibratory conveyor that discharges the aligned fibers onto the ribbon.

16. The method of claim 15 wherein a powdered glass is added to the aligned short fibers prior to the fusion of the ribbons together.

17. The method of claim 16 wherein the powdered glass is added to the vibratory conveyor.

18. The method of claim 1 wherein the atmosphere between ribbons is controlled by purging the space between the ribbons with a gas stream.

19. The method of claim 1 wherein the atmosphere between said ribbons is controlled by the application of 1 References Cited UNITED STATES PATENTS 220,908 10/1879 Arbogast 6550 10 OShaughnessy 6522 Dewey et al. 15689 Eakins et a1. 654

Martel l17-16 Hessinger et a1. 6518 Byrnes 6598X Singer 65-4 Jackson l6l193X S. LEON BASHORE, Primary Examiner R. L. LINDSAY, JR., Assistant Examiner US. Cl. X.R. 

